Cardiac myocyte hypertrophy is an essential chronic adaptation, but it eventually becomes maladaptive resulting in the transition from hypertrophy to failure. Our long term objective is to identify mechanisms by which alterations in myofilament protein structure/function relations promote the hypertrophic response and the transition from compensated to decompensated hypertrophy. There is compelling evidence that modifications of the thin filament regulatory proteins, cardiac troponin T (cTnT) and cardiac troponin I (cTnI), either by protein kinase C (PKC) phosphorylation or by switching to a mutant isoform, may be important in the transition between compensated hypertrophy and decompensation. Phosphorylation of cTnT and CTnI depress myofilament force generation and may be maladaptive. Our overall objective is to test the hypothesis that isoform switching of cTnT to mutant forms genetically linked to familial hypertrophic cardiomyopathy (FHC) alters functional effects of PKC dependent phosphorylation of both TnT and cTnI. Our specific aims address the following questions: Aim number 1. What is the relative significance of TnT PKC sites -Thr206, Thr 215, and Thr 295 - on Ca2+-dependent activation of myofilament force, crossbridge kinetics, and economy? Aim number 2. Are functional effects of cTnT or cTnI phosphorylation amplified or repressed in myofilaments containing cTnT-deltaexon 15,16 (missing 27 C- terminal amino acids including Thr 295)? Aim number 3. How is myofilament Ca2+-signaling involving protein-protein interactions of cTnT with CTnI and with cTnC altered by protein phosphorylation? These studies will provide important information relevant to our understanding of heart failure.